Enhanced structural uniformity of aluminum based alloys by thermal treatments



Dec. 30. 1969 Filed Jun. 21) 1967 EAR/N6 PCT. DIRECT/0N Gr/MM M. J.PRYOR l- 3,435,947 ENHANCED STRUCTURAL UNIFORMI'I'Y OF ALUMINUM BASEDALLOYS BY THERMAL TREATMENTS 2 Shuts-Shut 1 40 o I J r I I 2 2 If l l flJ, J

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INVENTORS. MICHAEL J. PR YOR FIG -4 v R/CHARDJ. SLUSAR ATTORNEY Dec. 30,1969 PRYQR ETAL 3,486,947

ENHANCED STRUCTURAL UNIFORMI'I'Y OI ALUMINUM BASED ALLOYS BY THERMAL,TREATMENTS Filed June 21. 1967 2 Shuts-Shut. I

a 2 E E 3% E I 3 R U Q m 0 I l I l g 6 7 8 9 M, H mvmons c AE JPRYOR E IR/CHA/PDJ SLUSAR United States Patent 3,486,947 ENHANCED STRUCTURALUNIFORMITY 0F ALUMINUM BASED ALLOYS BY THERMAL TREATMENTS Michael J.Pryor, Woodbridge, and Richard J. Slusar, North Haven, Conn., assignorsto Olin Mathieson Chemical Corporation, a corporation of Virginia FiledJune 21, 1967, Ser. No. 647,736 Int. Cl. C21d 1/78, 1/26 US. Cl. 14811.5

ABSTRACT OF THE DISCLOSURE This invention is directed to a method oftreating aluminum base alloy castings containing from about 0.5 to 2.0%manganese in order to obtain fine grain size and very low earingcharacteristics comprising: heating the castings at a temperature of 950to 1150 F. for 5 to 25 hours, cooling at a rate of between50 and 200 F.per hour to a temperature of from 700 to 950 F., hot working from to 70%within a temperature rangeof from 700 to 950 F., reheating the workedarticle to 950 to 1100" F. and holding within this temperature range for5 to 25 hours, cooling at a rate from 50 to 300 F. per hour to 700 to950 F., hot rolling at least 75% at a temperature in the 700 to 950 F.temperature range, cooling to room temperature, annealing at atemperature from 550 to 800 F. for a period of time of from one minuteto 24 hours, cooling to room temperature, cold rolling at least 10%, andannealing to recrystallize the alloy.

In the processing of aluminum alloys containing from 0.5-2.0% manganese,the molten metal is directly cast into ingots. In ingots of commercialsize, a microstructure as follows is obtained. The primary aluminumprecipitates out of the liquid first until a eutectic valley is reached,where precipitation of the eutectic aluminum and (Mn, Fe)Al occurs.Eutectic precipitation continues until a peritectic point is reachedwhere, according to the equilibrium diagram, a peritectic reactionshould take place.

However, under the non-equilibrium conditions existing in commercialcasting, the peritectic reaction is almost totally suppressed.Furthermore, the eutectic which forms after the primary aluminum hasprecipitated exhibits divorcement, that is to say, the aluminum portionof the eutectic precipitates on the previously precipitated primaryaluminum, leaving the (Mn, Fe)Al to solidify separately. Because of thecooling rate imposed by ingot size, the structure consists of a seriesof large dendritic cells, often 25 microns and greater in diametercontaining the primary aluminum as a core, outlined by the (Mn, Fe)Al-Al eutectic.

It is conventional to preheat and hold such manganese containingaluminum alloys at elevated temperatures prior to hot rolling in orderto obtain a more uniform structure.

This treatment, generally known as the preheat treatment, is carried outbetween 950-1150 F. to equalize short range compositional gradients,promote completion of unfinished solid state reactions and obtain a moreuniform structure within the ingot, by the process of diffusion. Uponcooling the ingot from the preheating tem- 8 Claims 1 3,486,947 PatentedDec. 30, 1969 perature, precipitation of the manganese constituents takeplace which, ideally would be uniform precipitation of the manganeseconstituents is not obtained in this step, a banded structure willdevelop during subsequent hot working. This banded structure cannot beeliminated by coldworking and annealing, and will result in coarse andvariable grain size, together with high earing percentages in subsequentforming operations. Thus, the structural conditions during preheatingwhich lead to the 0 development of this banded structure must beavoided.

Since precipitation of manganese constituents involves a sluggishdiffusion reaction the times required for banding elimination by preheattreatment alone are abnormally long. Furthermore, with very thick castingots and, therefore, very coarse dendrite spacings, diffusion distancebecomes unusually long. Effective thermal treatment in the absence ofadditional thermo-mechanical steps for obtaining structural uniformityat these temperatures, therefore, requires unusually long holding timefor 20" thick commercial ingot of from 50l00 hours. Such long holdingtimes are not economical commercially.

Several attempts have been made previously to shorten the hightemperature holding period by subsequently employing various auxiliarythermal and thermal-mechanical treatments. Typical process sequences aredescribed in US. Patents Nos. 3,249,491, 3,219,492 and 3,304,208. All ofthe processing described in this art involves shorter high temperatureholding times which are invariably succeeded by very slow cooling atrates less than 50 F./ hour to the hot rolling working temperature whichis normally within the range of 700-950 F. for these alloys.

Even with these very slow cooling rates extremely fine grain size,smaller than 8,000 grains/mm. is not always containing alloy productshaving fine grain size and low earing characteristics.

Itis another object of this invention to treat thick chill-cast aluminumalloy ingots containing Mn having a coarse divorced eutecticmicrostructure in order to obtain wrought products which exhibit finegrain size and low earing characteristics.

It is another object of this invention to process aluminum base alloyscontaining manganese to deep drawing products having a fine grain sizeand low earing characteristics with greatly foreshortened hightemperature holding times so that the processing is commerciallyfeasible.

It is another object of this invention to process aluminum base alloyscontaining about 0.5 to 2.0% manganese to deep drawing products having afine grain size and low earing characteristics with greatly shortenedhigh temperature holding times and relatively rapid cooling rates to thehot working temperature so that the processing is commercially feasible.

Other objects will appear from the following description and claims.

The following process, which is economical commercially, has been foundto achieve the additional objects of avoidance of banding, fine grainsize and low earing characteristics. The process is as follows:

(1) Heat the alloy at a temperature of 950 to 1150 F. for to 25 hours;

(2) Cool at a rate of between 50 and 200 F. per hour to a temperature offrom 700 to 950 F.;

(3) Hot work from to 70% within a temperature range of from 700 to 950F.;

(4) Reheat to 950 to 1100 F. and hold at this temperature for 5 to 25hours;

(5) Cool at a rate from 50 to 300 F. per hour to 700 to 950 F.;

(6) Hot roll at least 75% at a temperature in the 700 to 950 F.temperature range;

(7) Cool to room temperature;

(8) Anneal at a temperature from 550 to 800 F. for a period of time fromone minute to 24 hours;

(9) Cool to room temperature;

(10) Cold roll at least 10%;

(ll) Anneal at 550-800 F. one minute to 24 hours to ensurerecrystallization;

(12) Additional cold rolling and annealing treatments may be providedsubsequently as desired in order to give the desired gage.

It can be seen from FIGURE 1 that dendritic cell size generated incasting has a strong influence on earing. Curves 1 and 2 are for acommercial size ingot in which the dendritic cell size was from 25 to 40microns. Curve 3, however, is for a laboratory cast ingot with aconsiderably smaller dendritic cell size of less than 10 microns. It isapparent that if the dendritic cell size is small, as is the case incurve 3, earing is considerably less than in curves 1 and 2 where thedendritic cell size is larger. It is apparent from FIGURE 1 that if lowearing (below 1%) is to be obtained, the large commercial cell size mustbe efiiciently broken up.

Furthermore, the curve demonstrates the tendency of reduced earing withlower values of the (Mn+Fe):Si ratio.

FIGURE 2 is in the form of two photomicrographs of alloy 3003 containing1.06% mn, 0.57% Fe and 0.23% Si. FIGURE 2A the banded structure whichresults from a single hot rolling step at 850 F. following a preheattreatment of ll50 F./5 hrs. and 50 F./hr. cooling. This is to becontrasted with FIGURE 2B wherein after identical preheat treatment, hotrolling was performed at 850 F. for only a 30% reduction, followed by areheat treatment to 1100 F. for six hours and subsequent cooling to 850F. at a fast cooling rate of 100 F. per hour followed by a second hotrolling step at 850 F. totaling 75 reduction. It is apparent that FIGURE2B shows that the banding has been almost completely eliminated and thatthe structure is much more uniform, with good precipitate dispersion.

FIGURE 3 illustrates the marked effect of the preheat treatment ofFIGURE 28 followed by additional hot working upon the earingcharacteristics of the fully fabricated sheet as compared to one heattreatment at the times and temperatures specified in curves 1 and 2. Itis apparent that the caring is considerably less with the re-heating at1000 F. for six hours shown in curve 3. Furthermore, the generaltendency of reduced earing at lower ratios of (Mn+Fe) :Si is also to beobserved. The (Mn+Fe):Si ratio must be less than or equal to 7.5 toobtain earing values below 1%. By comparison with Curve 3 of FIGURE 1,it is believed that the low earing in curve 3 of FIGURE 3 is due to asmaller effective dendritic cell size and more uniform structureresulting from the re-heating and additional hot rolling.

FIGURE 4 shows the effect of the reheat treatment of FIGURE 2B followedby additional hot rolling upon the grain size of fully fabricatedsheets. It is apparent that the grain size is considerably smaller (moregrains per cu. mm) than with the single pre-heat treatment. FurthermoreFIGURE 4 shows that when using rapid cooling 4 rates of from 50-200 F./hr. after the preheat and rates of 50300 F./hr. after the reheat finegrain sizes even in excess of 20,000 mm. can be obtained. These veryfine grain sizes are to be contrasted with the best and much coarservalue of 8736 grain mm. disclosed in US. Patent 3,219,491 which couldonly be obtained with very slow and uneconomical cooling rate of lessthan 50 F./hour.

The cooling rate in step 2 is very important. The cooling rate cannot befaster than 200 F. per hour, because if it is, the earingcharacteristics and grain size are adversely affected. It is believedthat this is because manganese (and possibly iron) precipitation andgrowth will not take place if the cooling rate is greater than 200 F.per hour, but this is not certain.

However, the ratio must be at least 50 F. per hour or the time requiredto cool in shop processing becomes uneconomical. Not only the cost offuel and other operational expenses for the furnace must be consideredbut also, the furnaces are tied up for long periods of time and cannotbe used to process other material which must be shipped to customers ontime.

In step 5, the cool from the reheat temperature of 950 F.l F. at a rateof at least 50 F. per hour but not greater than 300 F. per hour again isvery important. The rate must again be slow enough to insure additionalprecipitation and growth of the manganese (and possibly iron). On theother hand, the rate must be fast enough for economical commercialproduction. The cooling rate in this second cool can be somewhat fasterthan the first cool, however, because the original dendritic cellstructure is largely broken up and diffusion distances for additionalstructural uniformity are considerably less.

Step 7 specifies a cool to room temperature which may be at anyconvenient rate, for example, at a rate of 5 0 F. per hour. However, itis not essential that this step take place.

If desired, the rolled product may be taken directly to step 8, which isan anneal at 550 to 800 F. for at least one minute and preferably for atime of from one to two hours. This post-hot-line-anneal has a verydefinite affect on both grain size and earing, as can be seen from TableI.

Table I shows the affect of this post-hot-line-anneal at 650 F. for 2hours, prior to further cold rolling and annealing, on earing and grainsize properties of material processed by our treatment. It is apparentthat with an anneal at 650 F. for 2 hours, earing and grain size aresubstantially reduced in all cases.

TABLE I Grain Size in Earing percent at 45 gr./mm.

Anneal Anneal No (650 F./ No (650 F./ anneal 2 hr.) anneal 2 hr.)

Gauge:

The observed differences in earing and grain size are believed to resultfrom a precipitation which takes place during the anneal. Furthermore,the grains which were elongated following the second hot rollingoperation have recrystallized and are now equiaxed.

After the anneal, the product is cooled to room temperature, at anyconvenient rate, for example, at a rate of approximately 5-0-100 F. perhour.

The product is then cold rolled and the cold rolling has a considerableaffect on grain size, as can be seen from FIGURE 4. It is apparent thatthe extent of prior cold rolling reduces the annealed grain size per se,and, furthermore, that the combination of the two-step hot rolling,interspersed with a reheat treatment at 950ll F., results in an evenfiner grain size. Grain size values of above 11,000 grains per cubicmillimeter may be obtained with 70% reductions.

After this cold roll, there must be an anneal, which is to be carriedout at 550-800 F. for a period of time of one minute to 24 hours, toensure recrystallization.

If desired, two or more cycles of cold rolling and annealing may takeplace, depending on the desired finished gage and the grain sizedesired.

Also, for wrought alloys when combined with a low temperatureprecipitation treatment subsequent to the hot line fabrication, theprocess is exceptionally effective in grain size refinement.

The following examples illustrate the invention without limiting itsscope.

EXAMPLE I A single 3003 aluminum alloy was commercially cast using thedirect chill method of casting. The ingot, consisting of 1.06% Mn, 0.57%Fe, 0.23% Si and 0.13% Cu was cast under plant conditions to a finishedingot cross section of 20" x 40". Casting drop rate was such that thedendritic cell size ranged from 25 to 40 microns in diameter. 3

A transverse slice from the ingot was reduced to 3" x 4 x 6 pieces. Twoof the pieces were designated A and B and given the following treatment.

Piece A was preheat treated at 1050" F. for 25 hours prior to being hotrolled to sheet thickness. Piece B received an 1150 F./ 5 hour preheattreatment. Both were cooled at the approximate rate of 85 F. per hour tohot rolling temperature.

Hot rolling of each slice was done at a temperature of 750 F. 25 F.50 F.to 0.500". The 0.500" plate was allowed to cool to ISO-200 F. tosimulate finish rolling and the plate rolled to 0.160" sheet. Uponcooling to room temperature the sheet was given a post-hot-linecoilanneal at 650:L- F. for 4 hours with a heat-up rate controlled at 50F./hour to temperature. Subsequent to the post-hot-line-anneal, eachsheet received a 70% nominal cold reduction in thickness and a finalanneal at 650 F. for 4 hours. Again, the heat-up rate for the finalanneal was controlled at 50 F./hour.

Earing and grain size results for the finished .050" (nominal) sheet aregiven below in Table II. It will be seen that these treatments (A and B)are ineffective in grain refining and earing reduction.

A third portion of the ingot (hereafter ingot C) was processed asfollows. Ingot C received a preheat treatment identical to ingot B.However, following the initial 30% reduction in thickness during hotrolling, the slab was reheated to 1050 F. for 6 hours. Upon cooling atthe rate of approximately 100 F. per hour to the original hot rollingtemperature of 725 F. 25 F., hot rolling and further fabricationcontinued as indicated for ingots A and B.

The test results given in Table II show that the invention (Ingot C) washighly effective in reducing grain size and earing.

TABLE II.-EARING AND GRAIN SIZE RESULTS AT .050"-0 TEMPER Grain sizeEaring height 6 EXAMPLE II An ingot of 3003 aluminum alloy consisting of1.16% Mn, 0.63% Fe, 0.19% Si, 0.15% Cu was commercially cast using thedirect chill method of casting. The ingot had a cross section size of19.5" by 40". The dendritic cell size ranged from 25 to 40 microns indiameter. The ingot was heated at 1125 F. for five hours. It was cooledto hot rolling temperature at a rate of F. per hour and it was then hotrolled at a temperature of 850 F. to a reduction of 30%. It was thenreheated to a temperature of 1018 F. and held for 6.4 hours. It wascooled to a second hot rolling temperature of 820 F. at a rate of 100 F.per hour and it was hot rolled to an 80% reduction. The so-rolledproduct was cooled to room temperature, annealed at 650 F. for one hour,cooled to room temperature, cold rolled 70% to .050" and then annealedat 650 F. for two hours. Following this treatment, the product had agrain size of approximately 7,500 grains per cubic millimeter and had anearing height percent of approximately 0.5%. This product was found tobe satisfactory for deep drawing both from a standpoint of earing andgrain size. A second cold rolling operation of 70% to .015 followed byan anneal yielded a product having a grain size of 12,500 gr./mm. andwith earing height of 0.3%

It is to be understood that the invention is not limited to theillustrations described and shown herein, which are deemed to be merelyillustrative of the best modes of carrying out the invention, and whichare susceptible to modifications of form, size, arrangement of parts anddetail of operation. The invention rather is intended to encompass allsuch modifications which are within the spirit and scope of theinvention, as set forth in the appended claims.

What is claimed is:

1. A process for treating cast aluminum alloys containing manganese inthe range from about 05-20% by weight comprising:

(A) heating the ingots at a temperature of 950 to 1150 F. for 5 to 25hours.

(B) cooling the ingots at a rate of between 50 to 200 F. per hour to atemperature of from 700 to 950 F. (C) hot working the ingots from 10 to70% within the temperature range of 700 to 0 F. (D) reheating the workedingots to 950 to 1100 F. and holding within this temperature range for 5to 25 hours, (B) cooling at a rate from 50 to 300 F. per hour to 700 to950 F.,

(F) hot rolling at least 75% in the 700 to 950 F.

temperature range,

(G) annealing at 550 to 800 F. for at least one minute,

(H) cooling to room temperature,

(1) cold rolling at least 10%, and then (J) annealing the cold rolledproduct.

2. A process according to claim 1 in which the casting is done by thedirect chill method.

3. A process according to claim 1 in which the aluminum alloy is 3003aluminum.

4. A process according to claim 1 in which more than me cold rollingstep, interspersed with an anneal, is carried out at the end of theprocess.

5. A process according to claim 1 in which low earing and a fine grainsize are obtained in the product.

6. A process according to claim 3 in which cold rolling is carried outto a reduction of at least 70% and a grain size of at least as fine as10,000 grains per cubic millimeter is obtained.

7. A process according to claim 5 in which cold rolling .is carried outto a reduction of at least 70% and a grain size of at least as fine as10,000 grains per cubic millimeter and earing of not greater than 1% isobtained.

8. A process according to claim 1 in which the as-cast dendritic cellsize averages at least 25 microns.

References Cited UNITED STATES PATENTS 5 2,262,696 11/1941 Nock et a1.148-115 3,219,491 11/1965 Anderson et a1. 14811.5

I. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner

